Ferroelectric materials are a class of materials that can be thought of as having electrical properties somewhat analogous to the magnetic properties of ferromagnetic materials. A uniaxial ferromagnetic material can be magnetized in one of two directions, and thereafter will retain a magnetic field in that direction even after the applied magnetic field is removed; similarly, a ferroelectric material can be “polarized” in either direction (by applying an electric field to it), and thereafter will retain an electric field in that direction, even after the applied electric field is removed.
Ferroelectric materials have been successfully integrated into integrated circuit processes, but this integration can have some drawbacks. Ferroelectric materials having sufficient thermal stability for integrated circuit processing often include incompatible metals that must be separated from a silicon substrate. Such ferroelectric materials also tend to be strong oxygen sources, increasing the risk of undesirable oxidation of adjacent materials. Additionally, ferroelectric materials generally can only withstand a finite number of polarization reversals before their performance degrades.
Ferroelectric memories exploit the properties of ferroelectric materials. These materials are useful in semiconductor memories as they have characteristics to provide a non-volatile memory function; after a ferroelectric material has been polarized in one direction, it will hold that polarization for an extended time without further power input. In contrast, dynamic random access memory (DRAM) requires periodic refresh to maintain its data value, thus losing its data value upon the removal of its power source.
Since the physics of ferroelectric floating-gate memories are similar to standard floating-gate memories (such as Flash memories), the sensing operation is correspondingly similar. Typically, floating-gate memories are sensed by detecting the activation/deactivation of the selected transistor in response to a given gate/source voltage. Although a typical floating-gate memory's activation/deactivation state is dependent on a stored charge of its floating gate, and a ferroelectric floating-gate memory's activation/deactivation state is dependent on a polarization of a ferroelectric layer, they both can exhibit this binary behavior.
At the microscopic scale, the ferroelectric material can be seen to be divided into domains. A domain is a volume within which the polarization of the material is uniform. Each domain can have only two stable polarization states. The magnitude of the polarization state of the bulk material is a composite of the individual domain polarization states.
FIG. 1 schematically shows a typical hysteresis curve 12 for a ferroelectric material. When the applied electric field E is increased to a positive value E1, the polarization of the material will increase to a value P1. When the applied positive field is subsequently removed, the polarization will fall back to a positive “remanent polarization” value Pr. In a similar manner, when the applied electric field is increased in the opposite direction, to a negative value −E2, the polarization of the material will go to a negative value −P2. When the applied negative field is subsequently removed, the polarization will fall back to a negative remanent polarization value −Pr. Thus, the material can take either of two polarization states in the absence of an electric field, depending on how it has been affected by the previously applied field. For electrical circuit analysis, the polarization state of a ferroelectric film can be thought of in terms of surface charge density, i.e., as amount of charge per unit area (usually written as “τ”). Curve 14 is an example of a minor hysteresis curve obtained when the same material is cycled between electrical potentials having insufficient magnitude to cause complete reversal of the polarization.
When an increasingly strong electric field is applied to a ferroelectric material, more and more of the domains will change their state to line up with the applied field. The electric field seen by any one domain is affected by the polarization states of the other domains which are nearby. Consequently, a full reversal of polarization requires not only some threshold energy level, but also some delay as individual domains align. This is inconvenient for ferroelectric memories, since it limits the write speed of any such memory. Moreover, in memories that use a destructive read, i.e., a read operation using a voltage sufficient to cause reversal of polarity, this phenomenon is also an important constraint on read access time as the data must be rewritten after sensing. This has been a problem with commercialization of ferroelectric memories, since it is highly desirable for ferroelectric memories to have access times approximately as fast as those for DRAM memories.
For the reasons stated above, and for other reasons stated below that will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for alternate architecture and methods of operation of ferroelectric semiconductor memory devices.